GB1590100A - Heart stimulation devices - Google Patents

Description

PATENT SPECIFICATION

o ( 21) Application No 45828/77 ( 22) Filed 3 Nov 1977 ( 31) Convention Application No.

738 418 ( 32) Filed 3 Nov 1976 in ah ( 33) United States of America (US) It ( 44) Complete Specification published 28 May 1981 ( 51) INT CL 3 A 61 N 1/36 1/08 ( 52) Index at acceptance A 5 R 85 D 6 ( 72) Inventor ROBERT LEE CANNON ( 11) ( 19) ( 54) IMPROVEMENTS IN OR RELATING TO HEART STIMULATION DEVICES ( 71) We, MEDCOR INC, a corporation organised under the laws of the Commonwealth of Massachusetts, U S A, of 5920 Rodman Street, Hollywood, Florida 33023, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:-

The present invention relates generally to heart stimulation devices, and particularly to atrial-synchronous heart stimulation devices Heart stimulation devices are commonly referred to as heart pacers.

In the field of medical electronics, heart pacers are becoming widely accepted and well known There are several varieties of heart pacer available on the market today.

There are pacers which provide stimulation to the ventricle of the heart at a fixed rate, pacers which provide stimulation to the ventricle of the heart on demand, pacers which provide stimulation to the atrium and ventricle of the heart in a predetermined timing sequence and on demand, and pacers which provide stimulation to the ventricle in response to signal inputs received from the patient's body other than from the ventricular chambers of the patient's heart (This listing is not intended to include all types of pacers available today) It is with this last mentioned type of heart pacer that the present invention is more particularly related.

In normally functioning heart, the sinoatrial mode initiates depolarization of the electrically polarized muscalature of the heart and serves as the normal pacemaker of the heart The depolarization initially causes contraction of the atria, as represented by the electrical P-wave, and subsequently causes contraction of the ventricles (represented by the QRS complex) following a delay known as the atrio-ventricular (A-V) delay The A-V mode is responsible for the A-V delay which ensures proper asynchronous contraction of the atria and ventricles for efficient cardiac hemodynamics In fact, the A-V delay comprises by far the major portion of the time between the P and R waves which also includes the time required to excite the atria and the time occupied by conduction from the A-V mode to the ventricular myocardium However, the A-V delay is not constant at all heart rates 55 Rather, it varies from a relatively long interval (i e 170-250 milliseconds) for slow heart rates (i e 55-70 beats per minute) to a relatively short interval (i e 125-170 milliseconds) for rapid heart rates (i e 130-150 60 beats per minute), with the A-V delay in the adult human heart tending to be greater than that in a child's heart at any particular heart rate A pacer according to an embodiment of the invention is particularly suited 65 for use when a so-called A-V block prevents normal conduction of the depolarizing voltage from the atria to the ventricles.

Detection of P-waves representing atrial contractions of the heart has been proposed 70 previously to control the timing of ventricular stimulation pulses Pacers utilizing P-wave detection may be generally designated as being atrial-synchronous U S.

Patent 3,648,707 which issued on March 14, 75 1972 is cited as a somewhat limited example of an atrial synchronous pacer That patent describes a device which can detect P-waves and which causes a ventricular pulse generator to supply a stimulation pulse at a 80 fixed period of time (A-V interval) after the P-wave detection occurred If no P-wave occurs, the pacer provides a stimulation to the ventricle of the heart at the end of a predetermined period of time from the pre 85 vious ventricular heart beat or stimulation.

If a P-wave is sensed at or near the time of occurrence of the ventricular stimulation impulse, the ventricular stimulation impulse still occurs at the expected time This results 90 in an upper limit on the interval between the ventricular stimulation impulses, or in other words, a lower limit to the rate of operation of the pacer.

More recently, there has been developed 95 a heart pacer which responds to sensed P-waves and includes timing and control means for generating a fixed-interval A-V interval pulse which (unless inhibited by a natural ventricular contraction) acts, at its 100 1590100 1 590 100 conclusion, to generate a ventricular stimulation impulse even if the P-wave occurs only short before the expiration of the predetermined basic timing interval initiated by the previous ventricular beat or stimulation.

This pacer is described in our co-pending U.K Patent Application 40180/77, (Serial No 1586268) which description is incorporated herein by reference This latter pacer is designed such that the basic timing interval provides a practical intermediate or median stimulation rate, which stimulation rate may decrease for relaxation or sleep, and alternatively increase for exercise, as a function of the rate of atrial contractions.

Even though the pacer described in our copending application represents a significant advance in heart pacers by its ability to vary the ventricular stimulation rate as a function of the atrial beating rate, it does not exactly simulate the heart's conduction system because it relies on an A-V interval of fixed duration independent of heart rate.

The use of a constant A-V interval may also tend to limit the practical range of heart rates over which the atiral synchronous pacer is operable.

In accordance with the invention there is provided an improved atrial-synchronous eart stimulation device for stimulating the heart of a patient, said device comprising terminals for connection to said patient's heart, timing means for establishing successive timing intervals, a pulse generator for generating a ventricular stimulation impulse on said terminals at the end of each said timing interval to effect a beating action of the ventricle of said heart, first means for detecting the beating action of the atrium of said heart, second means responsive to the beating action of the atrium detected by said first means so as to generate a pulse whose duration corresponds to a desired atrioventricular interval, said timing means being arranged to establish a basic interval thereby to provide said timing interval and including control means responsive to said pulse for varying said timing interval relative to said basic interval within a predetermined range such that said timing interval is limited to be no longer than the sum of said basic interval and the duration of said pulse if the otherwise next generated one of said ventricular stimulation impulses would have occurred during the time of occurrence of said pulse, and resetting means responsive to at least said ventricular stimulation impulse for resetting said timing means so as to commence a successive timing interval.

said second means including means responsive to successive beating actions of the atrium of said heart detected by said first means so as to control automatically the duration of each said pulse as a function of the interval between preceding successively detected atrial beating actions, thereby to control the respective timing interval.

The circuit for resetting the timing means is preferably responsive to the beating action of the heart's ventricle, which occurs 70 either naturally (demand operation) or in response to the ventricular stimulation impulse.

The pulse whose duration corresponds to a desired atrio-ventricular interval (A-V 75 interval pulse) is preferably generated by a circuit responsive to variations in the interval between successive atrial beating actions to vary the interval of the respective A-V interval pulse in the same time-sense direc 80 tion.

In a preferred embodiment, the A-V interval pulse generating means comprises a monostable multivibrator having first RC timing circuitry for determining the normal 85 maximum duration of the A-V interval pulse and second RC timing circuitry for determining the minimum time for full recovery of the monostable between successive A-V interval pulses of the normal max 90 imum duration, the monostable being triggered by each detected atrial beating action.

The A-V interval pulse is of less than the maximum duration if the respective triggering of the monostable occurs sooner than 95 completion of the full recovery following termination of the preceding A-V interval pulse and the first and second RC timing circuits are preselected to provide an A-V interval pulse of first duration when the 100 interval between successive atrial beating actions is of a first value, and of second duration shorter than the first duration when the interval between successive atrial beating actions is of a second value shorter 105 than the first value The first value of the interval between successive atrial beats may correspond with a low-limit heart rate (e g.

55-60 b p m) and the second may correspond with a high-limit heart rate (e g 110 140-150 b p m) The variation in the interval of the A-V interval pulse is substantially continuous across the heart rate range between the high and low limits.

The control circuitry for varying the 115 pacer's timing interval from the basic interval preferably comprises circuitry for lengthening the timing interval beyond the basic interval if the otherwise next generated one of the ventricular stimulation pulses would 120 have occurred during the time of occurrence of the A-V interval pulse and for limiting the lengthened interval to be no longer than the sum of the basic interval and that of the A-V interval pulse and alternatively for 125 shortening the timing interval to less than the basic interval if the detected beating action of the heart's atrium occurs within a predetermined interval which ends prior to the end of the basic interval by the length or 130 1 590 100 interval of the A-V interval pulse.

In order that the present invention be more readily understood, an embodiment thereof will now be described, by way of example with reference to the accompanying drawings, in which:Figure 1 illustrates the voltage wave form produce by the heart during one complete heart beat; Figure 2 comprises a sequential graphical representation of the voltage, relative to a threshold voltage, appearing across a timing capacitor of a relaxation oscillator used in a previously proposed heart pacer; Figure 3 comprises graphical representations similar to Figure 2, though not necessarily in contiguous sequence, showing the threshold voltage modified in accordance with one mode of operation of an embodiment of the present invention; Figures 4 A and 4 B comprise graphical representations similar to Figure 2, though not necessarily in contiguous sequence, showing the threshold voltage modified in accordance with another mode of operation of an embodiment of the present invention; Figure 5 is a block diagram illustrating a preferred embodiment of the pacer of the invention; Figure 6 is a more detailed circuit diagram of portions of the pacer illustrated in Figure 5; Figure 7 is a detailed schematic diagram of the monostable multivibrator comprising the interval control for providing A-V interval pulses in which the respective intervals are P-wave-rate-dependent; and Figure 8 is a graphical time plot of the voltage waveforms occurring at several significant locations in the A-V interval control monostable of Figure 7.

The human heartbeat is represented electrically as a complex wave consisting of what are designated P, Q, R, S and T waves as shown in Figure 1 The P-wave represents an atrial beat associated with atrial'depolarization, which beat commands the heart rate as a function of signals from the rest of the body The major and most pronounced electrical pulse in the heart is the R-wave which represents ventricular contraction Typically the R-wave has an amplitud 3 relation to the P-wave of at least 3:1, although it will be appreciated that the P-wave may be of equal or greater magnitude than the R-wave if both are sensed in the atrium The R-wave normally represents depolarization of the ventricles, but when, due to some cardiac malfunction, it does not occur, it is a function of the artificial pacer to provide periodic electronic pulses to the heart to stimulate contraction If both the heart and artificial pacer supply an R-wave, however, competition for control of the heart results and a possibly dangerous situation arises when the pacer electronic pulse occurs in a T-wave region, or so called "vulnerable period" To avoid this problem, there has been developed a so-called demand pacer as typified in U K Patent No 1,266,907, the 70 subject matter of which is incorporated herein by reference.

Referring to Figure 5, there is illustrated in diagrammatic form an embodiment of an artificial pacer constructed in accordance 75 with the invention and having a catheter l operatively connected thereto for positioning electrode poles El and E 2 in the atrium 2 and electrode poles E 2 and E 3 in the ventricle 4 of heart 6 The catheter 1 is inserted 80 into the heart 6 until its electrode pole E 3 which is on the tip of the catheter 1 lodges in the apex of the ventricle 4 Electrode pole E 2 in the ventricle 4 is adjacent but spaced from electrode pole E 3, and similarly elec 85 trode pole E 2 in the atrium 2 is adjacent but spaced form electrode pole El The spacing between electrode poles El and E 3 is such that when pole E 3 contacts the apex of ventricle 4 the electrode pole El is spaced 90 therefrom in the atrium 2 This spacing is typically about 11 centimeters This degree of spacing between electrodes El and E 3 is preferred in order to maximize the sensing of the P-wave Myocardial implantation of 95 the electrodes will, of course, provide a suitable alternative, as will the use of separate transvenous electrodes.

Electrode pole El is connected by conductor 500 to the input of a P-wave detector 100 520 for conveying sensed P-waves thereto.

Similarly, the ventricular QRS complex sensed by electrode pole E 3 is conveyed by conductor 11 to the input of an R-wave detector 510 Conductor 11 additionally 105 extends to the output of pulse amplifier 550 of the pacer from which it receives the ventricular stimulation pulses applied to ventricle 4 by means of electrode pole E 3 Electrode poles E 2 in both the atrium 2 and the 110 ventricle 4 are connected electrically in common to conductor 9 to provide a common or "ground" electrical reference level connected to some respective portions of the pacer The R-wave detector 510 and 115 P-wave detector 520 are capable of providing pulses indicative of the detection of a QRS complex and a P-wave respectively.

While it may be possible to use various types of P-wave and R-wave detectors, these 120 detectors in the preferred embodiment are constructed as disclosed in U K Patent No.

1,266,907 The present R-wave detector 510 is substantially identical to the QRS complex detection circuitry and rate dis 125 criminator stage of U K Patent No.

1,266,907.

The P-wave detector 520 is essentially the same as the R-wave detector 510, each detector including band-pass filters, how 130 1 590 100 ever the filters associated with the amplifying stages of P-wave detector 520 are scaled to have a somewhat higher pass-band center frequency than the filters of the R-wave detector (e g 80 Hz vs 40 Hz), to optimize their sensitivity and response to the P-wave and QRS complex respectively as discussed in our aforementioned copending Application 40180/77 (Serial No 1586268) which is incorporated herein by reference.

Ventricular timing circuit 540 is operative in a free running or continuously operating mode to provide pulses VSP recurring at a regular or so-called "basic" interval, as illustrated in Figure 2, those pulses being amplified by pulse amplifier 550 and applied to electrode E 3 through conductor 11 for stimulation of the ventricle 4 The ventricular timing circuit 540 essentially comprises a relaxation oscillator having a basic timing interval in the free running mode and which is inhibited by a reset pulse on conductor 580 from R-wave detector 510 each time an R-wave appears on conductor 11, as required for "demand" operation A variable interval controller 530 for adjusting or controlling the timing of ventricular timing circuit 540 forms part of the general pacer timing circuitry and is connected to the ventricular timing circuit 540 by a conductor 570 The controller 530 is responsive to pulses appearing on conductor 560 from the output of P-wave detector 520 each time an atrial P-wave is detected for altering the A-V interval.

Through the appropriate selection of components and their values in interval control circuit 530 and in conjunction with the circuitry of ventricular timing circuit 540, the pacer is capable of adjusting the timing of the production of a pulse from circuit 540 such that the ventricular stimulation impulses normally occur following an automatically variable delay or interval (termed the A-V interval) after the occurrence of a detected atrial P-wave The interval between successive ventricular stimulation pulses may be increased and/or decreased relative to the regular or basic interval in accordance with a respective decrease and/or increase in the atrial P-wave rate.

Further, the duration of the A-V interval is automatically determined as a function of the heart rate as measured by the detected P-waves.

The basic timing interval of ventricular timing circuit 540 may be established as that providing a stimulation pulse rate of about 75 beats per minute, and the P-wave rate may then act to slow this rate as for relaxation or sleeping and additionally may act to accelerate the rate as during exercise Even if the P-wave detection and interval controlling circuitry is ineffective, as due to the absence of atrial P-waves or a failure in that processing circuitry, the pacer will continue to operate as a conventional demand pacer at a stimulation rate determined by the rate of the circuit 540 ( 75 beats per minute, 800 70 ms intervals) which is clearly adequate for life support and well within the upper and lower limits.

Reference is now made to Figure 6 which is a partially diagrammatic schematic of the 75 illustrative embodiment of the present invention The schematic of Figure 6 is to be viewed in conjunction with Figure 1 of the aforementioned U K Patent No 1,266,907, the combination providing an operative 80 embodiment except for the inclusion of P-wave detector 520 Accordingly, to the extent that U K Patent No 1,266,907 is incorporated for its disclosure of R-wave detector 510, it is similarly incorporated for 85 the disclosure of a detector such as P-wave detector 520 which differs from the R-wave detector 510 in only the relatively few ways mentioned above The following components of Figure 6 of the instant application 90 are identical to components of Figure 1 of the aforementioned Patent specification:

the transistors T 6, T 7, T 8 and T 9; resistors 35, 37, 55, 61, 63 and 59; capacitors 57 and 65; electrodes E 2 and E 3; and conductors 9 95 and 11 Interconnection of these components is described in the aforementioned Patent specification Other components in

Figure 6 of the instant application may be equivalent to components in Figure 1 of the 100 aforementioned Patent specification, but are given different reference numerals.

In addition to that material disclosed in U.K Patent No 1,266,907 which is incorporated herein by reference, certain other 105 portions of the pacer as illustrated in Figure 6 are described in U S Patent No.

3,774,619 to which reference may be made for additional information Specifically, batteries B 1, B 2, B 3, B 4 and B 5, the parallel 110 connected compensating diode 104 and resistor 108, the resistor 109 in series with resistor 108 and the parallel-connected capacitor 111 and resistor 110 in series with resistor 109 The junction of resistors 109 115 and 110 (junction J) is connected to the base of transistor T 7 and the potential of this junction is the threshold level of the timing circuitry Certain other elements of Patent No 3,774,619 have been omitted 120 inasmuch as they do not affect the substance of the present embodiment.

The P-wave detector 520 and interval control 530 are operatively connected to junction J of the pacer timing circuitry to 125 adjust the threshold level potential in response to the detection of atrial P-waves in heart 6 The interval control 530 as illustrated in Figure 6 includes a monostable multivibrator or one-shot 600 designed for 130 1 590 100 generating an A-V interval pulse of automatically variable duration within predetermined limits in response to a trigger pulse received from the output of P-wave detector 520, as will be explained thereinafter in greater detail A variable resistor 610 enables the monostable 600 to be preset to provide a nominal A-V delay pulse having an interval typically equal to or greater than 250 milliseconds.

The A-V interval pulse 620 at the output of monostable 600 is of rectangular waveform and may, in one mode of operation of the embodiment of the invention, be connected substantially directly to junction J In another mode of operation, the A-V interval pulse 620 is fed to junction J through a capacitor 630 to obtain a preselected "droop" in the pulse waveform voltage applied to junction J While these two modes of operation might normally appear in separate pacers, in the interest of brevity they have been combined in Figure 6 and illustrated as being available in a single pacer through use of a presettable single pole, double throw switch 640.

One pole of switch 640 is connected to the output of monostable 600 and the other pole is selectively connectable either to one end of capacitor 630 (as shown) or to terminal 650 of a bypass conductor connected to the conductor 570 and the other end of capacitor 630.

The capacitor 630 is shown as being variable to illustrate that the extent of droop introduced to A-V delay pulse 620 may be varied as will be described.

Interrelating the circuitry of the instant application with U K Patent No 1,266,907, conductor 140 is connected to the junction to resistors 29, 31 and 33 in that patent specification Conductor 580 is connected to the side of capacitor 53 remote from the side which is connected to the junction of capacitor 49 and resistors 47 and 45 in that Figure In this illustrative embodiment of the present invention, switch S of Fig 1 of that patent specification is omitted (i e consider switch S to be held closed at all times).

For brevity, magnetic field responsive circuitry for disabling detectors 510 and 520 has been omitted from the drawings but may comprise a desirable option.

Briefly considering the operation of components 104 and 108-111 in the illustrated pacer, current from the series string of batteries flows through resistor 59 and charges up capacitor 65 which holds the voltage as long as transistor T 9 is not turned on Current from the series string of batteries also flows through the parallel series circuit including the parallel combination of diode 104 and resistor 108, series resistor 109 and the parallel combination of capacitor 111 and resistor 110.

If the heart requires a stimulating impulse, transistors T 7 and T 8 cause transistor T 9 to conduct, causing capacitor 65 to discharge through ventricular electrodes E 2, E 3, and the heart 6 The transistors T 7 and 70 T 8 comprise a relaxation oscillator which is timed by the rate at which capacitor 57 charges (trace 270 in Figs 2-4) to the threshold voltage level appearing at the base of transistor T 7 (junction J) When the vol 75 tage on capacitor 57 is such that the timing voltage 270 on the emitter of transistor is equal to that on its base (junction J), the relaxation oscillator is triggered, causing transistor T 9 to conduct When transistors 80 T 7 and T 8 conduct, capacitor 57 is discharged and thus reset Transistor T 6 is connected across timing capacitor 57 and is responsive to a pulse on conductor 580 from capacitor 53 each time an R-wave is 85 detected for conducting and thereby also discharging and resetting capacitor 57 In this manner, detected R-waves act to reset the relaxation oscillator before completion of a basic interval ( 800 ms) or cycle in order 90 to prevent a ventricular stimulation impulse from being generated during the vulnerable period following a ventricular contraction.

When T 8 conducts, some of the current for the collector of T 8 comes from the 95 charged capacitor 111 After T 8 stops conducting capacitor 111 recharges to its former state through resistors 108 and 109.

Capacitor 111 is chosen so that it does not recharge to its previous static voltage value 100 in a time equal to or less than the time between pulses in the ordinary heart rate to provide what is known in the prior art as rate hysteresis.

The P-wave detector 520 responds to 105 each P-wave by generating a trigger pulse 561 which is applied to the triggering input of monostable 600 via conductor 560 The input to monostable 600 is connected to one side side of a coupling capacitor 601 which 110 is analogous to capacitor 53 in the R-wave detection circuitry of U K Patent No.

1,266,907 While the trigger pulse 561 applied to monostable 600 is herein illustrated as being positive it will be appreciated 115 that a pulse of opposite polarity mught be provided if required.

Referring to Fig 7, the monostable 600 comprising the automatic A-V interval control of the stimulator of the present inven 120 tion is illustrated in greater detail The general configuration of monostable multivibrator 600 may be any of several general types and is here shown as being of the collector-coupled transistor type in which a 125 pair of NPN transistors T 20 and T 21 have their emitters connected in common to the same potential, here denoted as Vo and corresponding with an intermedate voltage from the battery-series B 1-B 5 As is well 130 1 590 100 known, a monostable multivibrator has a stable state and a metastable state In the illustrated embodiment, T 20 is biased "off' or in non-conduction and T 21 is "on" or in saturation in the stable state.

The respective collectors of transistors T 20 and T 21 are connected to a voltage V+ relatively more positive than Vo at their emitters through resistors 604 and 605 respectively The base of T 21 is cross coupled to the collector of T 20 through timing capacitor 606 The base of T 20 is cross coupled to the collector of T 21 through resistor 607 A biasing resistor 608 connected between the base of T 20 and a voltage V relatively more negative than Vo at its emitter serves, with resistor 607, to form a voltage divider for biasing T 20 off A timing resistor 610 connected to the junction of capacitor 606 and the base of T 21 extends to the voltage also applied to resistors 604 and 605.

Resistor 610 may be variable for selecting and presetting a nominal duration for A-V interval pulse 620 A pair of oppositelypoled steering diodes (not shown) biased by resistor 608 might extend from coupling capacitor 601 to the respective bases of T 20 and T 21 so that an incoming trigger pulse of either polarity might be used to switch monostable 600 from its stable state to the metastable state.

Referring to Fig 7 and the waveforms of Fig 8, when a positive trigger pulse 561 is applied to the base of T 20 at t=o, the high positive voltage VW at the collector of T 20 begins to fall (becomes less positive or more negative) This negative going voltage is coupled to the base of T 21 and the forward bias is decreased The base current and collector current of T 21 begins to decrease.

The collector voltage Vc 2 of T 21 increases positively A portion of this voltage is coupled through resistor 607 to the base of T 20, increasing its potential Vb W positively Thisregeneration results in a rapid change of both T 20 and T 21 Since capacitor 606 was initially charged to a potential almost equal to the voltage V+ appearing on the common terminals of resistors 604, 610 and 605, the base of T 21 is now at a negative potential almost equal to the magnitude of the difference between Vo and V+.

Capacitor 606 discharges through resistor 610 and the low saturation resistance of T 20 The base potential Vb 2 of transistor T 21 becomes less negative Generally speaking, when the base potential of transistor T 21 becomes slightly positive of the cut-off voltage VCO at t=T, T 21 again conducts; the collector potential of T 21 increases negatively and is coupled to the base of T 20 driving it into cut-off; and transistor T 20 is again at cut-off and T 21 is in saturation with its collector voltage almost at Vo This stable condition is maintained until another pulse triggers the circuit The A-V interval pulse (Vc 2) is taken from the collector of T 21 and its normal maximum duration as a relatively positive rectangular waveform is primarily determined by the 70 RC time constant of resistor 610 and capacitor 606 during the above mentioned discharge of the capacitor The magnitude of the A-V interval pulse may be variably preset by replacing resistor 605 with a 75 potentiometer or series pair of resistors and deriving the pulse at the wiper or junction.

The cut-off voltage VCO for the bases of T 20 and T 21 is slightly below VO.

The normal duration of A-V interval 80 pulse 620 is represented by the interval from t=o to t=T in Fig 8 This assumes, however, that the recovery timing circuit including resistor 604 and the capacitor 606 has had sufficient time to return the mono 85 stable 600 to a quiescent stable state before the next trigger pulse, according to the generally preferred mode of operation of monostables in various applications including prior art pacers (See pages 184-185 and 90

600-602 of pulse and Digital Circuits, by Millman and Taub, McGraw Hill, 1956).

However, the time constants of the A-V interval and of the recovery circuits of monostable 600 are selected to be of such 95 duration that trigger pulses recurring at the rates associated with heart rates in the range of 55-140 beats per minute operate to reduce the duration of A-V interval pulse 620 from that normal or maximum duration 100 obtained when successive trigger pulses occur at intervals greater than the sum of the normal maximum duration plus the recovery time This reduction in the normal duration of the output pulses is, in other 105 instances and applications, considered undesirable and is explained in greater detail at the aforementioned pages in Pulse and Digital Circuits.

Briefly, the voltage at the base of T 21 110 rises exponentially toward V+ just after a trigger pulse and experiences an overshoot when T 21 goes on and T 20 goes off.

Further, the capacitor 606 must recharge through resistor 604 in series also with the 115 much smaller resistance r of the emitterbase circuit of T 21 In order that the normal maximum duration of the output pulse of monostable 600 not be disturbed, the overshoot on the base of T 21 must fully decay 120 and the capacitor 606 must fully recharge to place the collector of T 20 to V+ Both these functions require the same time, the so called recovery time, which depends on the time constant (R+r)C, where R is the value 125 of resistor 604, r is the much smaller value of the emitter base resistance of T 21, and C is the value of capacitor 606.

If a subsequent trigger pulse designated 561 ' in Fig 8, occurs before completion of 130 1 590 100 the minimum recovery time, the subsequent change in voltage Vci at the collector of T 20 is less than "normal" The main result is that the following sudden negative decrease in the voltage Vb 2 at the base of T 21 is correspondingly less than normal, such that capacitor 606 then begins to discharge through resistor 610 at a voltage relatively closer to V+ than normal, thereby shortening the interval during which T 21 does not conduct to provide the A-V pulse 620 of less than normal maximum duration.

In the present embodiment, the values of resistor 610 and capacitor 606 are selected such that the normal maximum duration of A-V interval pulse 620 is at least about 225-250 milliseconds (i e 300 ms) Further, the values of resistor 604 and capacitor 606 should be selected to provide a relatively long recovery time For example, the normal maximum A-V interval of at least 225-250 milliseconds may be selected to correspond with heart rates below about 58 beats per minute, and increases in the heart rate up to about 140 beats per minute should result in corresponding reductions in the duration of the A-V interval pulse to a limit of about 125-150 milliseconds.

It will be appreciated that the normal maximum duration of the output pulse ( 620) from monostable 600 could conceivably be as much as 700-900 milliseconds, but the values of resistors 604 and 610 and capacitor 606 should be such that for trigger rates (i e sensed P-wave or heart rates) of about 55-60 per minute the duration of the output pulse 620 would be shortened to about 225-250 ms with attendant further continuous shortening to about 125-150 ms for respective further heart rate increases to about 140 per minute.

Generally speaking, the reduction in the interval as a function of the increase in heart rate over the contemplated operating range generally bears the relationship of about 1:1 or 1:12, and an appropriately slopeld section of the exponential recovery time constant curve should be selected for operation By making the value of resistor 604 relatively large, the several hundred-ohm value of base-emitter resistance r may essentially be omitted in determining the (R+r) C recovery time constant.

It will be further appreciated that if the normal maximum interval of the output pulse of monostable 600 is relatively long (i.e 500-1000 ms), a relatively short recovery time constant may suffice; whereas if the normal maximum interval of the monostable is relatively short (i e 250-300 ms) the recovery time constant will need to be at least as long as the remainder of the 1000-1025 ms interval which corresponds with a heart rate of 60-55 beats per minute.

It is preferable that triggering of monostable 600 be conducted in that portion of the recovery time constant curve having a relatively small slope in order to minimize the change in the A-V interval during alternate triggerings at any constant trigger rate as 70 described at pages 185-186 of Pulse and Digital Circuits This consideration suggests, then, that the normal maximum interval of the output pulse from monostable 600 be relatively near 250 ms to allow a long recov 75 ery time constant with subsequent triggering occuring during the "flatter" later portion thereof.

Having discussed the novel means by which the interval of A-V interval pulse 620 80 is varied as a function of sensed atrial heart rate, the resulting pulse 620 is then extended to the junction J at the base of T 7 to adjust the threshold voltage VT thereat in accordance with the disclosure in our co 85 pending application 40180/77 (Serial No 1586268) If pulse 620 is connected directly to junction J (excluding or bypassing capacitor 630), the voltage VT is elevated by that of the pulse 620 throughout its duration 90 such that the ventricular stimulation pulse (VSP) timing interval may be extended, as illustrated in Fig 3, beyond the free running interval illustrated in Fig 2 The ventricular pulse timing interval may be extended by as 95 much as the interval of the A-V pulse 620, if the latter begins (as the result of P-wave, P) just before time-out of the basic 800 ms timing interval of the relaxation oscillator In this somewhat restricted embodiment, ven 100 tricular stimulation pulses, VSP, may occur (assuming no inhibition by natural ventricular contractions) at intervals in the range between 800 ms and 800 ms plus the interval of A-V pulse 620 For instance, if it is 105 desired to provide stimulation pulses VSP in response to P-waves sensed at a rate as low as 55-60 per minute ( 1025-1000 ms interval), the interval of pulse 620 at that rate would be about 210-225 ins Should the 110 P-waves be occurring at a rate of 70 per minute ( 858 ms interval), the monostable 600 is scaled such that the interval of pulse 620 at that rate is about 190-200 ins It will be appreciated that this arrangement, 115 although allowing "slow-down" of the pacer from a basic rate, is limited in range ( 75 to 60-55 beats per minute), and thus would not require monostable 600 to vary the interval of pulse 620 more than about 50-75 ms The 120 right-most portion of Fig 3 illustrates the situation of an increased P-wave rate, and thus shortened A-V interval ( 130 ins), in which the adjustment to VT is unable to control the generation of pulse VSP 125 In a preferred arrangement, the A-V interval pulse 620 is extended through capacitor 630 to the junction Jat the base of T 7 By coupling pulse 620 through capacitor 630, the threshold potential at junction J is 130 8 1 590 100 8 rapidly increased by the magnitude of pulse 620 and then begins to decay or "droop" as the capacitor 630 begins to charge at a rate determined by the time constant of the circuit through the capacitor and thence through the several parallel paths comprising serial resistors 108 and 109, resistor 110 and capacitor 111 The rate of droop is preset by appropriate selection of values of the Waforementioned circuit components and particularly capacitor 630 For instance, the less the capacitance of capacitor 630, the greater its charging rate and thus, the greater the rate of droop It will be realized that the actual magnitude (voltage) of droop is a function of the duration of A-V pulse 620, which value is variable according to the present invention Further, the aforementioned capacitor 111 included for purposes of rate hysterisis might be omitted, with an appropriate rescaling of capacitor 630 and the impedance of its charging circuit to retain the desired "droop" Such rescaling of capacitor 630 and its charging circuit might also be controlled to retain a degree of rate hysterisis, if desired.

Through this introduction of "droop" to the adjusted threshold potential VT at junction J it is possible to not only extend the ventricular pulse interval beyond the basic 800 ms rate, but also to reduce the interval to less than 800 ms for atrial beat rates which exceed 75 per minute; and which may be as great as 140-150 per minute This may be seen in Fig 4 wherein the first instance (Fig 4 A), the atrial beat rate is about 55-60 per minute, the P-wave, P, occurs at about 790-795 ms into the ventricular timing interval and the A-V interval pulse is of about 225 ms duration in accordance with the atrial rate and where in the second instance (Fig 4 B), occurring during a period of exercise, the atrial beat rate is about 140 per minute, the P-wave, P occurs at about 290 ms into the ventricular timing interval and the A-V interval pulse is of about 140 ms duration in accordance with the present atrial beat rate The magnitude (voltage) and rate of the "droop" in the adjusted threshold potential VT must be sufficiently great that the sudden decrease in potential VT at the termination of the A-V pulse intersects the rising timing voltage 270 for atrial beat rates as great as 140-150 per minute, however, it must not be so great that, when the A-V pulse is of 225 ms duration at atrial rates of 55 beats per minute, it prematurely intersects the rising timing voltage 270 before completion of the A-V interval The surest way to accomplish this and ensure the range of required "droop" magnitudes is with an A-V pulse of relatively large magnitude Further, the RC time-constant of the circuit through which capacitor 630 charges should be less than the RC time-constant of the circuit through which the timing capacitor 57 charges in order to avoid the situation in which the P-wave occurs particularly early in the escape interval and the adjusted threshold 70 voltage does not intersect the timing voltage on its downstep at the completion of the 200 millisecond A-V interval but sometime thereafter when the relatively "slower" recovering threshold voltage is met by the 75 more rapidly rising timing voltage 270.

Although not specifically illustrated in Figs 2-4, it will be appreciated that the occurrence of an R-wave is overriding and will inhibit the generation of a stimulation 80 pulse by resetting the relaxation oscillator and thereby beginning anew the basic timing interval.

Claims (8)

WHAT WE CLAIM IS:-

1 An atrial-synchronous heart stimula 85 tion device for stimulating the heart of a patient, said deivce comprising terminals for connection to said patient's heart, timing means for establishing successive timing intervals, a pulse generator for generating a 90 ventricular stimulation impulse on said terminals at the end of each said timing interval to effect a beating action of the ventricle of said heart, first means for detecting the beating action of the atrium of said heart, second 95 means responsive to the beating action of the atrium detected by said first means so as to generate a pulse whose duration corresponds to a desired atrio-ventricular interval, said timing means being arranged to establ 100 ish a basic interval thereby to provide said timing interval and including control means responsive to said pulse for varying said timing interval relative to said basic interval within a predetermined range such that said 105 timing interval is limited to be no longer than the sum of said basic interval and the duration of said pulse if the otherwise next generated one of said ventricular stimulation impulses would have occurred during 110 the time of occurrence of said pulse, and resetting means responsive to at least said ventricular stimulation impulse for resetting said timing means so as to commence a successive timing interval, said second means 115 including means responsive to successive beating actions of atrium of said heart detected by said first means so as to control automatically the duration of each said pulse as a function of the interval between 120 preceding successively detected atrial beating actions, thereby to control the respective timing interval.

2 A heart stimulation device according to claim 1 wherein the duration of a said 125 pulse varies in the same time-sense direction as a variation in the respective interval between the immediately preceding successively detected atrial beating actions.

3 A heart stimulation device according 130 1 590 100 1 590 100 to claim 2 wherein said second means comprises a monostable multivibrator, said multivibrator having first R-C timing means for determining the normal maximum duration of said pulse and second R-C timing means for determining the minimum time for full recovery of the monostable between successive said pulses of maximum duration, said monostable multivibrator being triggered by each detected atrial beating action, a said pulse being of less than said maximum duration if the respective triggering of said multivibrator occurs sooner than completion of said full recovery following termination of the preceding said pulse, and said first and said second R-C timing means being preselected to provide a said pulse of first duration when the interval between successive atrial beating actions is of a first value, and of second duration shorter than said first duration when the interval between successive atrial beating actions is of a second value shorter than said first value.

4 A heart stimulation device according to claim 3 wherein said first value of the interval between successive atrial beating actions corresponds with a low-limit heart rate and said second interval value corresponds with a high-limit heart rate.

5 A heart stimulation device according to claim 4 wherein variation in the duration of said pulse in the heart-rate range between said low-limit and said high-limit is substantially continuous across said range.

6 A heart stimulation device according to any one of the preceding claims wherein said resetting means includes means for detecting the beating action of the ventricle of said heart and means responsive to said last-mentioned detecting means for resetting said timing means.

7 A heart stimulation device according to any one of the preceding claims wherein said control means for varying said timing interval comprises means for lengthening said timing interval beyond said basic interval if the otherwise next generated one of said ventricular stimulation impulses would have occurred during the timing of occurrence of a said pulse and for limiting said lengthened interval to be no longer than the sum of said basic interval and the duration of said pulse and alternatively, for shortening said timing interval to less than said basic interval if said detected beating action of the atrium occurs within a predetermined interval ending prior to the end of said basic interval by said length of said pulse.

8 A heart stimulation device substantially as hereinbefore described with reference to Figures 3 to 8 of the accompanying drawings.

A A THORNTON & CO, Chartered Patent Agents, Northumberland House, 303/306 High Holborn, London, WCIV 7 LE.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd, Berwick-upon-Tweed, 1981 Published at the Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies maybe obtained.